Put insert's summary line back on insert

[jonassaa]

One line moved. has_walk was inserted between insert's doc comment and
insert in #479, so the comment was swallowed:

    /// File a freshly prepared walk, evicting the least recently used one.
    /// Whether a walk for `key` is already filed, whatever it can answer.
    ///
    /// Not the same question as `first_page`, ...
    pub fn has_walk(...)

    pub fn insert(&self, repo: &RepoId, key: WalkKey, order: Arc<WalkOrder>) {

Two consequences. has_walk's rustdoc summary line — the one line that
shows in generated docs, in an IDE hover and in a symbol list — read "File a
freshly prepared walk, evicting the least recently used one", which describes
insert and is the opposite of what has_walk does: it counts no hit and
moves nothing. And insert, the function that actually evicts, was left
undocumented.

Why nothing caught it. It compiles, every test passes, and no content was
lost — both sentences were present, attached to the wrong functions. The only
tell is reading the file, which is how it was found: diffing the merged branch
against main to prove the content had landed byte-identically.

The cause is worth naming, because the tool makes it easy: anchoring an
insertion on pub fn insert puts the new item above that line, and a Rust
doc comment sits above the signature it documents — so an insert anchored on a
signature lands inside the preceding item's documentation. Anchor on the
doc comment's first line, or on the blank line before it.

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